core, chdir: collapse slashes, avoid copy-to-self

core, chdir: collapse slashes, avoid copy-to-self

Collapse multiple slashes into one (this still doesn't resolve . and
.. in the path, since that requires awareness of symlinks.)

This code also avoids a copy-over-self bug by introducing a temporary
buffer.

Reported-by: Gene Cumm <gene.cumm@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>